Surveillance illuminator systems using arrays of LEDs mounted on metal heatsinks are widely used in the security industry to provide visible or infrared (IR) illumination for CCTV cameras, imaging or data capture devices. LED illuminators are claimed to offer lifetimes in excess of 100,000 hours, however their effective output decays from the moment the LEDs are activated. Lifetime output reductions of 20% to 50% have been quoted in manufacturer's data, and LED output is often specified at 50% of the maximum operating current at a particular ambient temperature. The need for increased illuminator range and output necessitates that LEDs be driven to their current limits in surveillance applications, a practice that reduces illuminator effectiveness, reliability, and operational lifetime.
Factors that can also degrade LED output and lifetime of surveillance illuminator systems include, but are not limited to: operation of LED arrays at fixed output currents; operation at high ambient temperatures which reduces LED efficiency (even when constant current power supplies are used); production variances in LED die quality, LED efficiency, and inefficient lensing can represent a variation of up to +/−20% in optical output power between different illuminator units.
Known prior art surveillance illumination systems utilize and may combine illuminator feedback, heat sinking, and pulse width modulation to prolong LED lifetime. However, the lifetime of even the best of these systems is still limited by their high current operation, and high temperature operation due to their use of encapsulated LEDs. Less applicable prior art uses less reliable current sensor feedback instead of direct light sensor feedback to maintain nominal illuminator output. The effective range of prior art LED illuminators can vary dramatically with temperature and time, and from unit to unit. No prior art LED illuminator system produces surveillance images of sufficiently reliable quality over the maximum operational lifetime of the monitoring equipment.
A number of patents exist which utilize photo detectors to provide feedback on optical power output for LED arrays or other light sources. At least one patent (U.S. Pat. No. 6,028,694—Schmidt) uses pulse width modulation to increase LED light output for a given heat load. A number of patents or publications seek to provide constant or ‘stable’ brightness or optical power, often through the feedback provided by photo detectors. Other patents seek to ‘maximize’ optical output from the LEDs by altering the voltage or current. A patent application that combines a limited number of the features most relevant to the present invention is LED Array Package with Internal Feedback and Control by Mazzochette, et al (US 20060012986).